Voyager observations of solar wind proton temperature: 1‐10AU

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Abstract
Solar wind proton temperatures are measured simultaneously by the Voyager 1 and 2 spacecraft, far from Earth, and the IMP 8 spacecraft in Earth orbit. This technique allows the separation of radial and temporal variations of solar wind parameters. The average value of the proton temperature from 1 to 9 AU is observed to decrease as r−0.7 ± 0.2 which is slower than would be expected for adiabatic expansion. A detailed look at the solar wind stream structure shows that considerable heating occurs at the interface between high and low speed streams.